Daniel Samson
e854f65623
Every doc verified claim-by-claim against the code by parallel audit agents, then fixed and adversarially re-verified. Two waves of staleness corrected: the originally audited findings (higher-half boot handoff, kernel VFS takeover, fault isolation + claim release + driver restart, AML/S5 moving to ring 3, threading's shipped design, USB+FAT landing) and a second pass of adjacent claims the verifiers caught (smp.md 'not built yet' intro, system-requirements' PS/2-only and no-storage claims, halting.md's red-panic and no-IDT text, testing.md's serial mirroring, router-era vfs-protocol wording, capsule-first boot loading). threading.md now documents the shared-fate gap explicitly: the design says a process dies whole, the kernel today kills only the offending thread. Also fixes three stale code comments (isr.s exceptionHandler, acpi.zig sleepValue, build.zig boot-volume) — comments only, no behavior change.
79 lines
3.8 KiB
Markdown
79 lines
3.8 KiB
Markdown
# The kernel heap
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The [frame allocator](frame-allocator.md) hands out fixed 4 KiB physical frames;
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the [VMM](paging.md) maps pages into virtual addresses. The **kernel heap** sits on
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top of both to provide what the rest of the kernel actually wants: `alloc(n)` /
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`free(p)` for arbitrary byte sizes. It's the first real consumer of `map()`, and
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the thing that unlocks dynamic data structures — lists, hash maps, driver state,
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eventually a process table.
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It's generic kernel code (`system/kernel/heap.zig`): the allocator logic is
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architecture-neutral, using `architecture.mapPage` and the frame allocator underneath.
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## A growable free-list allocator
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The algorithm is a classic **first-fit free list**:
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- The heap owns a virtual region. Free space is tracked as an **address-ordered
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singly linked list** of free blocks; each block begins with a 16-byte header
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(`size`, and a `next` link used while free).
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- **alloc(n)** walks the list for the first block big enough. If the block is much
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larger it's **split** — the front becomes the allocation, the remainder stays
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free. If nothing fits, the heap **grows** (below) and the search retries.
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- **free(p)** finds the block header just before `p` and inserts it back into the
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list, **coalescing** with the physically adjacent free blocks on either side so
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the space can be reused as one region rather than fragmenting away.
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Allocations are 16-byte aligned; larger alignments aren't supported yet (the
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`std.mem.Allocator` `alloc` returns `null` for them).
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## Growing on demand
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The heap lives in the **higher half** of the address space (virtual base
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`0xFFFF_8000_0000_0000`) — unmapped, well clear of the low half, which belongs
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to user space (unmapped in the kernel's own tables; per-process user address
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spaces now map into it). (That base is x86_64-canonical; another architecture
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would pick its own.)
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When the free list can't satisfy a request, `grow` extends the mapped region: it
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pulls fresh frames from the [frame allocator](frame-allocator.md) and `map`s each
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onto the end of the heap, then adds the new span as a free block (coalescing with
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the current tail). So the heap starts at one page and expands page-by-page as
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demand requires, up to a cap. This is exactly what the VMM's on-demand `map` was
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built for.
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## A std.mem.Allocator
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The heap is exposed as a **`std.mem.Allocator`** (`heap.allocator()`), Zig's
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standard allocator interface. That's a deliberate multiplier: it means the whole of
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Zig's standard library — `ArrayList`, `AutoHashMap`, `std.fmt.allocPrint`, and the
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rest — works directly on the kernel heap, no bespoke containers required.
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## Verifying it
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The `heap` test (see [testing.md](testing.md)) exercises the allocator end to end:
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```
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[PASS] alloc 4096 bytes
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[PASS] heap memory is writable and reads back
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[PASS] freed block is reused <- free list + coalescing works
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[PASS] many allocations (heap growth) stay valid <- grow() maps fresh frames
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[PASS] std.ArrayList on the kernel heap <- std containers work on it
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```
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The "freed block is reused" check (free then re-alloc returns the same address) is
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the proof that free and the free list actually work, not just alloc; "heap growth"
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forces allocation past the initial page so `grow`/`map` runs; and the `ArrayList`
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check is the std-integration payoff.
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## What's next (largely still true)
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- **Thread/interrupt safety** — overtaken by the big kernel lock: SMP arrived
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with a single kernel lock taken at every kernel entry, which serializes all
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heap access. The heap still has no lock of its own, and needs none unless the
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big lock is ever split.
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- **Larger alignments** than 16 — still unsupported; page-aligned and DMA
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buffers come straight from the frame allocator instead.
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- **`resize`/`remap` in place** — still not done; growing an `ArrayList` copies.
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- **Reclaiming empty tail pages** — still not done; the heap only ever grows.
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